Improve MtlsPopTokenNotSupportedinImdsV1 error message clarity (#5908)

* Initial plan

* Update MtlsPopTokenNotSupportedinImdsV1 error message to be clearer and user-friendly

Agent-Logs-Url: https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/sessions/ccb50df4-2634-46bc-8072-d52f91b35139

Co-authored-by: gladjohn <90415114+gladjohn@users.noreply.github.com>

* Fix escaped apostrophe in MtlsPopTokenNotSupportedinImdsV1 error message

Agent-Logs-Url: https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/sessions/ccb50df4-2634-46bc-8072-d52f91b35139

Co-authored-by: gladjohn <90415114+gladjohn@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: gladjohn <90415114+gladjohn@users.noreply.github.com>

Author: Copilot

src/client/Microsoft.Identity.Client/MsalErrorMessage.cs

@@ -453,7 +453,7 @@ public static string InvalidTokenProviderResponseValue(string invalidValueName)
         public const string MalformedOidcAuthorityFormat = "Possible cause: When using Entra External ID, you didn't append /v2.0, for example {0}/v2.0\"";
         public const string InvalidCertificate = "The certificate received from the Imds server is invalid.";
         public const string CannotSwitchBetweenImdsVersionsForPreview = "ImdsV2 is currently experimental - A Bearer token has already been received; Please restart the application to receive a mTLS PoP token.";
-        public const string MtlsPopTokenNotSupportedinImdsV1 = "A mTLS PoP token cannot be requested because the application\'s source was determined to be ImdsV1.";
+        public const string MtlsPopTokenNotSupportedinImdsV1 = "mTLS Proof of Possession with managed identity is currently in private preview and is not supported on this VM. Ensure you're running on a supported VM image.";
         public const string ManagedIdentityAllSourcesUnavailable = "All Managed Identity sources are unavailable.";
     }
 }